In conventional slagging furnaces for steam generation and the like, solid carbonaceous fuels, such as crushed bituminous or semi-bituminous coal, are fed into a reaction or combustion zone. The combustion temperature is maintained at or above the ash fusion temperature in order to convert a major portion of the non-combustible ash present in the fuel to molten slag. Such furnaces have generally been relatively very large, having required the use of ceramic structures to prevent erosion of the combustion chamber by the higher temperature combustion products, have released large quantities of pollutants into the atmosphere, and have been severely limited as to the applications for which they could be utilized. The relatively large size of such furnaces has resulted in relatively high heat losses, lower overall thermal efficiency, and undesirably low power density, that is, thermal power output per unit of volume of the furnance.
One type of conventional slagging furnace is a so-called cyclone furnace, an example of which is disclosed in U.S. Pat. No. 2,357,301. In this cyclone furnace, crushed coal and oxidizer gas are both injected tangentially into the combustion chamber in a manner such that the gas, coal particles, and combustion products undergo high velocity cyclonic flow through the chamber. The non-combustibles present in the coal are centrifuged onto the combustion chamber wall to form a film of molten slag on the wall. A small quantity of relatively fine coal particles burn in their flight through the chamber while the vast majority of the coal is larger coal particles which are centrifuged onto the chamber wall. These larger particles adhere to the molten slag film on the wall and burn while on the wall.
In conventional cyclone furnaces this combustion of relatively-large, crushed-coal particles embedded in the slag on the combustion chamber walls tend to create a strongly reducing atmosphere in an annular region adjacent the walls. This is deleterious. Metal compounds (such as the iron oxides and sulfides commonly found in western American coals) are often reduced by this atmosphere thereby forming pools and puddles of liquid metal in the chamber-insulating slag layer. These puddles of liquid metal and sulfur compounds constitute, in effect, thermal short circuits through which extremely high heat flux may be conducted to the chamber wall, often resulting in overload of the cooling system and catastrophic melt-through of the metallic walls. In addition, the presence of sulfur compounds, with molten metal, may result in unacceptable corrosion phenomena.
One way of alleviating or avoiding the foregoing problems is to operate a conventional cyclone furnace with sufficient excess air to maintain an oxygen-rich stoichiometry near the combustion chamber wall. Operating a cyclone furnace with the large amounts of excess air required to avoid metal puddling and chemical corrosion is undesirable because doing so very significantly reduces the overall thermal efficiency of the boiler facility. In addition, a serious disadvantage of excess air combustion, in this age of stringent clean-air regulations, is that it results in excessive production of nitrogen oxides far exceeding the levels permitted by present day environmental standards.